Re: [PATCH v2 0/3] Enable THP for text section of non-shmem files

[Song Liu <songliubraving@fb.com>]

> On Jun 19, 2019, at 6:13 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> On Tue, 18 Jun 2019 21:48:16 +0000 Song Liu <songliubraving@fb.com> wrote:
> 
>>> I'm wondering if this limitation can be abused in some fashion: mmap a
>>> file to which you have read permissions, run madvise(MADV_HUGEPAGE) and
>>> thus prevent the file's owner from being able to modify the file?  Or
>>> something like that.  What are the issues and protections here?
>> 
>> In this case, the owner need to make a copy of the file, and then remove 
>> and update the original file. 
>> 
>> In this version, we want either split huge page on writes, or fail the 
>> write when we cannot split. However, the huge page information is only 
>> available at page level, and on the write path, page level information 
>> is not available until write_begin(). So it is hard to stop writes at 
>> earlier stage. Therefore, in this version, we leverage i_mmap_writable, 
>> which is at address_space level. So it is easier to stop writes to the 
>> file. 
>> 
>> This is a temporary behavior. And it is gated by the config. So I guess
>> it is OK. It works well for our use cases though. Once we have better 
>> write support, we can remove the limitation. 
>> 
>> If this is too weird, I am also open to suggestions. 
> 
> Well, it's more than weird?  This permits user A to deny service to
> user B?  User A can, maliciously or accidentally, prevent user B from
> modifying a file which user B has permission to modify?  Such as, umm,
> /etc/hosts?

I have removed this behavior in v3. I think we really don't need this. 

Thanks,
Song